1. Field of the Invention
The present invention relates to a positive electrode for a rechargeable lithium battery and a method of preparing the same, and more particularly, to a positive electrode for a rechargeable lithium battery having improved capacity, power, and cycle-life characteristics, and a method of preparing the same.
2. Description of the Related Art
Rechargeable lithium batteries use a material from or into which lithium ions are deintercalated or intercalated for positive and negative active materials. A lithium salt solution in an organic solvent or a polymer is used as an electrolyte. Rechargeable lithium batteries produce electric energy as a result of changes in the chemical potentials of the active materials during the intercalation and deintercalation reactions of the lithium ions. A rechargeable lithium battery having an average discharge voltage of approximately 3.7 V is considered to be one of the essential components in the digital generation since it is an indispensable energy source for portable digital devices, such as cellular phones, notebook computers, and camcorders (i.e., the so called “3C” devices).
For the negative active material in the rechargeable lithium battery, metallic lithium was used in the early days of development. Recently, however, since the metallic lithium causes problems such as a short life due to a high reactivity toward the electrolyte and the formation of lithium dendrites, carbon materials, such as amorphous carbon or crystalline carbon based materials, which reversibly intercalate lithium ions, are extensively used instead of the metallic lithium. With the use of carbon-based active materials, the battery performance has been improved tremendously. The short life and safety problems due to the metallic lithium has been prevented, relatively high energy density of the battery has been achieved, and cycle-life characteristics are also improved. It has also been suggested to add additives, such as boron, to the carbonaceous material, especially, by coating. For example, a boron-coated graphite (BOC) improves the performance characteristics of the carbonaceous materials.
For the positive active material in the rechargeable lithium battery, chalcogenide compounds into or from which lithium ions are intercalated or deintercalated are used. Typical examples thereof include LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoXO2 (0<x<1), and LiMnO2. A manganese-based positive active material, such as LiMn2O4 or LiMnO2, is readily prepared, is less expensive than the other materials, and is environmentally friendly. However, the manganese-based materials have a disadvantage of a relatively low capacity. LiNiO2 is inexpensive and has a high capacity, but is difficult to prepare in the desired structure. Among these materials, LiCoO2 is the most accepted in the battery market since its overall performance characteristics are better than the others. Accordingly, most of the current commercially available rechargeable lithium batteries (approximately 95%) use LiCoO2 as the positive active material, but it is rather expensive.
There is a great deal of effort being exerted to find an alternative thereof in order to reduce the cost of the active material. For example, in U.S. Pat. No. 5,292,601, LixMO2 (wherein M is an element selected from Co, Ni, or Mn; x is 0.5-1) is suggested as an alternative to LiCoO2 as a positive active material. U. S. Pat. No. 5,705,291 also discloses a method of fabricating a rechargeable lithiated intercalation battery including the operations of mixing a coating composition including boron oxide, boric acid, lithium hydroxide, aluminum oxide, lithium aluminate, lithium metaborate, silicon dioxide, lithium silicate, or mixtures thereof with a lithiated intercalation compound particulate, and fusing the coating compound at a temperature in excess of about 400° C. so as to coat the particulate with the fused coating compound.
However, there are continuing demands for further improved positive active materials especially for improvements in power and cycle-life characteristics.